Numerical Investigation of Variable Transpiration Cooling Effectiveness in Laminar and Turbulent Flows for Hypersonic Cruise Vehicles

Abstract

The thermal management of hypersonic air-breathing vehicles presents formidable challenges. Reusable thermal protection systems (TPS) are one of the key technologies that must be improved in order to use hypersonic vehicles as practical, long-range transportation systems. The active cooling systems, such as transpiration cooling, have to be considered for affordable, long duration flights to achieve efficient temperature reduction and coolant mass saving. With this technique, it is important to understand the physics that characterize the boundary layer and its interaction with the vehicle's surface. This paper investigates the effectiveness of a variable-velocity transpiration strategy for fully laminar, transitional, and fully turbulent flows over a 2-D blunt body with a cylindrical leading edge and a wedge region. The transitional flow cases are evaluated for a range of transition locations to see the influence of this parameter on the variable transpiration strategy introduced. For all flow types presented in this paper, a saw-tooth wall velocity distribution (variable transpiration strategy) is compared to a uniform-velocity transpiration approach. An equal amount of coolant usage has been imposed in order to compare the cooling effectiveness between both strategies for various flow types in different regions of the body. The results show that the uniform-velocity transpiration allows a reduction of 68% in the stagnation point heat flux and 77% for variable-velocity transpiration with respect to the no-transpiration case in both laminar and turbulent flow cases. The computational results show that the efficiency of the transpiration cooling is much higher in laminar flow compared to turbulent flow in regions downstream of the stagnation point. In such regions, for turbulent flows, the amount of total coolant must be increased by 110% (factor of roughly 2) to match the cooling efficiency observed in laminar flows. In addition to the analysis of cooling effectiveness, the thermal response of TPS material with the variable transpiration strategy is also investigated for both fully laminar and fully turbulent flows.A TOT = surface exposed to the flow, m 2 BC = boundary condition BL = boundary layer B 0 = permeability coefficient, m 2 C/C = carbon-carbon C p = specific heat at constant pressure, J/kg.K G = mass flow rate per unit of area, kg/m 2 .s H = material thickness, m k = thermal conductivity, W/m.K h V = heat transfer coefficient, W/m 3 .K = mass flow rate, kg/s = mass flux, kg/m 2 .s Nu = Nusselt number ODE = ordinary differential equation P = pressure, Pa q = heat flux, W/m 2 Re = Reynolds number r p = sphere radii that simulate porosity, m SP = stagnation point T = temperature, K TPS = thermal protection system t = time, s U = longitudinal velocity, m/s V = transversal velocity, m/s ∂ = partial derivative Δ = incremental ratio 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition 2 ρ = density, kg/m 3 ε = porosity η = cooling efficiency factor Subscripts CF = coo...